@ @YW@ @ÖïÐaínÜaì@Þî†bjnÜa@ @ .20 + 6 = 26 .C(7,2) − 1 = 7 ! − 1 = 20 4 2!×5! 500 6 .a = 1,2, 3, 4,5, 6 a3 +a3 4 [AIME 2005] ( ) . . . : . () .8 . : 4 .C(8, 4) . 9 ×C(8, 4) = 9 × 8 ! = 630 4!×4!